Magnesium hydroxide particles, process for producing the same, and resin composition containing the particles

ABSTRACT

Provided are magnesium hydroxide particles having a hexagonal crystal form and having an aspect ratio (H) which satisfies the following expression (I), 
     
       
         0.45· A·B&lt;H &lt;1.1· A·B   (I) 
       
     
     (wherein H is an aspect ratio, A is an average secondary particle diameter (μm) of all of the particles measured by a laser diffraction scattering method and B is a specific surface area (m 2 /g) of all of the particles measured by a BET method), a flame-retardant comprising the particles, a flame-retardant resin composition comprising 100 parts by weight of a synthetic resin and a 5 to 300 parts by weight of the magnesium hydroxide particles, and a molded article therefrom. The magnesium hydroxide particles are hexagonal single crystals, the hexagonal form thereof are not necessarily required to be regular hexagonal, and the size thereof are not necessarily constant. However, the aspect ratio thereof has a specific value in relation to the specific surface area and the average particle diameter, and the magnesium hydroxide particles have excellent properties as a flame retardant for synthetic resins.

FIELD OF THE INVENTION

The present invention relates to magnesium hydroxide particles having anovel form each and a process for the production thereof. Morespecifically, the present invention relates to magnesium hydroxideparticles having a hexagonal crystal form each and having a specificaspect ratio, and a method of the production thereof. Further, thepresent invention also relates to a flame retardant and a resincomposition containing the above magnesium hydroxide particles.

PRIOR ART OF THE INVENTION

Magnesium hydroxide particles have been known for a long time and areused for medical and industrial products in broad fields. For example,the medical products include an antacid, an evacuant, medicaments foranimals, etc., and the industrial products include a flame retardantwhich imparts a thermoplastic resin flame resistance when incorporatedinto the resin, an adsorbent for an oil-containing effluent, a sootdesulfurizer, a waste water neutralizer and a soil improver.

A variety of methods of producing magnesium hydroxide particles havebeen hitherto available, such as a method in which ion bitten, seawateror dolomite is used as an Mg source and lime or sodium-hydroxide is usedas an alkali source, a method which is based on the hydration ofmagnesium oxide and a method in which an Mg salt and ammonia are allowedto react deposit a magnesium hydroxide crystal. In conventionalmagnesium hydroxide, however, the form of magnesium hydroxide particlesis determined depending upon a production method, so that a differentproduction method has been required for obtaining a magnesium hydroxideparticles having a different form.

Meanwhile, an organohalogen compound or antimony trioxide is widely usedas a flame retardant for synthetic resins.

A halogen-containing flame retardant comprising an organohalogencompound, antimony trioxide or a combination of these generates a largequantity of smoke and toxic gases in fire, and it has therefore sociallycaused problems. Flame retardants have been therefore studied foravoiding the use of the halogen-containing flame retardant so as to makeits amount as small as possible. As a result, magnesium hydroxideparticles have come to be evaluated as an effective flame retardant.When magnesium hydroxide particles are incorporated into a resin, theamount of smoke during combustion is small, and they are non-toxic.Further, magnesium hydroxide particles can be applied to a broad rangeof resins since they are free from a phenomenon that those like aluminumhydroxide particles are dehydrated and decomposed at a resin processingtemperature to foam a resin molded article.

Synthetic resins containing a high concentration of magnesium hydroxideparticles as a flame retardant have come to be widely used in electriccables for use in/for an atomic power plant, vessel, an automobile, asubway and communications in an underground or tunnel passage, in a partfor an electric home appliance and an electronic machine or equipmentand in a construction material.

PROBLEMS TO BE SOLVED BY THE INVENTION

When a synthetic resin is flame-retarded with magnesium hydroxideparticles, the synthetic resin is required to contain a highconcentration of magnesium hydroxide particles, and there is caused aproblem that a poor appearance is formed or that properties of acomposition of the synthetic resin are deteriorated. For overcoming theabove problems, there have been proposed flame retardant of magnesiumhydroxide particles which are surface-treated with a higher fatty acidor various coupling agents. However, the above problems have not yetessentially solved.

The present inventors have therefore made diligent studies for obtainingnovel magnesium hydroxide particles having an unconventional specificform. It has been therefore found that magnesium hydroxide particleshaving a relatively large aspect ratio unlike conventional one andhaving a hexagonal crystal form can be obtained by adding a specificacid or a salt thereof in the step of producing magnesium hydroxideparticles from magnesium chloride or magnesium oxide as a raw material.According to the present invention, particularly, there are providedmagnesium hydroxide particles having an aspect ratio of a specific valuewith regard to a specific surface area and an average particle diameter.

In the present invention, studies have been further made concerning useof the above magnesium hydroxide particles having a specific form, andit has been found that the magnesium hydroxide particles have excellentproperties as a flame retardant for synthetic resins.

MEANS TO SOLVE THE PROBLEMS

According to the present invention, there is provided magnesiumhydroxide particles having a hexagonal crystal form and having an aspectratio (H) which satisfies the following expression (I),

0.45·A·B<H<1.1·A·B  (I)

wherein H is an aspect ratio, A is an average secondary particlediameter (μm) of all of the particles measured by a laser diffractionscattering method and B is a specific surface area (m²/g) of all of theparticles measured by a BET method, and a flame retardant comprising themagnesium hydroxide particles.

According to the present invention, further, there are provided aflame-retardant resin composition comprising 100 parts by weight of asynthetic resin and 5 to 300 parts by weight of the above magnesiumhydroxide particles, and a molded article produced therefrom.

The present invention will be explained further in detail hereinafter.First, the magnesium hydroxide particles having a hexagonal crystal formand the process for the production thereof, provided by the presentinvention, will be explained.

The magnesium hydroxide particles of the present invention arecharacterized by a hexagonal crystal form and a specific aspect ratio.The hexagonal crystal form is observed, for example, in a microscopicphotograph of the particles taken at a magnification of 5,000 to 10,000diameters. The magnesium hydroxide particles of the present inventionare hexagonal single crystals, and the hexagonal form is not required tobe of a regular hexagonal form. FIG. 1 shows a board having the form ofa regular hexagon as a model for calculation of an aspect ratio.However, the hexagonal form may be a form having six sides forming theboard as a total, and for example, may be a hexagon that the length ofeach of two sides opposed to each other is larger than the length ofeach of the other sides. Further, angles (interior angles) formingapexes of corners (six corners) formed by combinations of two sides incontact with each other may be 100° to 130° each. Further, the corners(six corners) formed by combinations of two sides in contact with eachother may be partly rounded. When observed in an enlarged photograph,most (at least 90%, preferably at least 95%) of the magnesium hydroxideparticles of the present invention have the above hexagonal crystalform, and the sizes of them are not necessarily uniform. However, thesizes (particles diameters) of the particles have a certain constantdistribution width as will be described later, and one of the featuresof the magnesium hydroxide particles of the present invention is thatthe sizes of the particles are relatively uniform. The distribution ofthe particle diameters will be further explained later.

The magnesium hydroxide particles of the present invention havecharacteristic features that the single crystal thereof is hexagonal andthat the aspect ratio (H) thereof is relatively large as compared withconventional ones. The range of the aspect ratio (H) is determined incorrelation with values of an average secondary particle diameter (A)and a BET specific surface area (B) of the magnesium hydroxideparticles. That is, the aspect ratio (H) is in the range which satisfiesthe following expression (I) with regard to a product (A×B) of theaverage secondary particle diameter (A) and the BET specific surfacearea (B).

0.45·A·B<H<1.1·A·B  (I)

The range of the aspect ratio (H) is preferably a range which satisfiesthe following expression (I-a), more preferably a range which satisfiesthe following expression (I-b).

0.50·A·B<H<1.1·A·B  (I-a)

0.55·A·B<H<1.0·A·B  (I-b)

In the above expressions, A is an average secondary particle diameter(μm) of the magnesium hydroxide particles and B is a specific surfacearea (m²/g) of the particles measured by a BET method.

When the aspect ratio (H) of the magnesium hydroxide particles issmaller than (0.45·A·B), the amount of particles having smaller aspectratios is relatively large, and the properties of the magnesiumhydroxide particles of the present invention are no longer obtained.Further, when the above aspect ratio is considerably small, the form ofthe magnesium hydroxide particles is no longer distinguishable from theform of conventional ones. When the aspect ratio (H) is larger than avalue of (1.1·A·B), undesirably, it is difficult to produce suchparticles stably, and it is difficult to mix or disperse such particleswith/in a resin uniformly when the particles are used as an additive forthe resin.

The magnesium hydroxide particles of the present invention also have acharacteristic feature in that they have a relatively narrow particlesize distribution as described above, that is, they are uniform inparticle size. This characteristic feature of the particle sizedistribution is represented by the fact that the volume ratio ofmagnesium hydroxide particles having a secondary particle diameter (F)satisfying the following expression (II), based on the total volume ofthe magnesium hydroxide particles, is at least 60%, preferably at least65%, particularly preferably at least 70%.

0.3·A<F<1.7·A  (II)

wherein F is a secondary particle diameter (μm) of the magnesiumhydroxide particles, and A has the same definition as that in the aboveexpression (I).

Desirably, the magnesium hydroxide particles of the present inventionhave an average secondary particle diameter (A), measured by a laserdiffraction scattering method, of 0.15 to 5 μm, preferably 0.5 to 3.0μm. With an increase in the average secondary particle diameter, thecontact area to a resin decreases so that thermal stability is improvedwhen they are used as an additive, for example, for a rubber, a ceramicor a resin. However, there are caused problems that mechanical strengthdecreases and that an appearance is poor. When the magnesium hydroxideparticles of the present invention are used as an adsorbent or aneutralizing agent in the form of a powder or granules, the workabilityis more improved with an increase in the average secondary particlediameter thereof. When they are used for medical products, theoccurrence of dust decreases and the workability is more improved withan increase in the average secondary particle diameter. When the averagesecondary particle diameter is too large, however, such particles aredifficult to administer orally and a preparation is difficult to make.

Advantageously, the average secondary particle diameter (A) as amagnesium hydroxide is therefore in the range of 0.15 to 5 μm,preferably 0.5 to 3.0 μm.

Further, the BET specific surface area (B) of the magnesium hydroxideparticles is 1 to 150 m²/g, preferably 2 to 130 m²/g, particularlypreferably 3 to 90 m²/g.

In the magnesium hydroxide particles of the present invention,preferably, all of (i) the aspect ratio (H), (ii) the average secondaryparticle diameter (A) and (iii) the BET specific surface area (B)thereof satisfy the above-described ranges.

The magnesium hydroxide particles of the present invention are suitablefor use as a flame retardant for synthetic resins owing to the above (i)aspect ratio (H), (ii) average secondary particle diameter (A), (iii)BET specific surface area (B) and the characteristic feature of theparticle size distribution. When the magnesium hydroxide particles areused as a flame retardant, they are incorporated in an amount ofapproximately 5 to 300 parts by weight, preferably approximately 10 to250 parts by weight, per 100 parts by weight of a synthetic resin. Whenthe magnesium hydroxide particles of the present invention are used as aflame retardant, the specific surface area thereof, measured by a BETmethod, is 30 m²/g or less, preferably 3 to 20 m²/g, particularlypreferably 3 to 10 m²/g. When a relatively large amount of magnesiumhydroxide particles are incorporated into a synthetic resin as describedabove, a molded article is sometimes deteriorated due to heating duringmolding or heating during the use of the molded article, so that themolded article is degraded in properties inherent to the molded article,such as impact strength, elongation and tensile strength. This is causedby chemical properties of magnesium hydroxide particles, a kind and acontent of impurities in particular, rather than by physical propertiesof the magnesium hydroxide particles.

According to studies by the present inventor, it has been founddesirable that the total content, as a metal content, of an ironcompound content and a manganese compound content as impurities in theparticles in the magnesium hydroxide particles of the present inventionis 0.01% by weight or less, preferably 0.005% by weight or less, inaddition to the above-described physical properties which the magnesiumhydroxide particles of the present invention have.

In the magnesium hydroxide particles of the present invention,preferably, the total content of (Fe+Mn) as metals is in the above rangeas described above. More preferably, the content, as a metal content, ofheavy metal compounds which also include a cobalt compound, a chromiumcompound, a copper compound, a vanadium compound and a nickel compoundis 0.02% by weight or less. That is, in the magnesium hydroxideparticles, more advantageously, the total content of(Fe+Mn+Co+Cr+Cu+V+Ni) as metals is 0.02% by weight or less, preferably0.01% by weight.

With an increase in the content of the iron compound and the manganesecompound in the magnesium hydroxide particles, the thermal stability ofa resin to which the magnesium hydroxide particles is incorporated iscaused to be more degraded.

As long as (i) the aspect ratio (H), (ii) the average secondary particlediameter, (iii) the BET specific surface area and (iv) the total contentof the iron compound and the manganese compound (or the total content ofthese and the other metal compounds described above) are within theabove ranges, the magnesium hydroxide particles are excellent incompatibility with a resin, dispersibility, moldability andprocessability and can give a resin composition satisfying an appearanceof a molded article, mechanical strength and flame retardancy and havinggood qualities.

For bringing the content of an iron compound and a manganese compound(and the other metal compounds as described above, as required) in themagnesium hydroxide particles into the above range, in the method ofproducing the magnesium hydroxide particles as will be described later,it is required to select those in which the content of these impuritiesis small, as magnesium chloride and magnesium oxide which are rawmarterials, and further, when an alkaline compound is used, it issimilarly required to use an alkaline compound having a high purity.Further, as materials for machines and equipment for a productionprocess, such as a reactor, a reserve tank, tubings, dryer, apulverizer, etc., it is required to use those from which the elution andinclusion of the above impurities are small.

The magnesium hydroxide particles of the present invention may betreated with a surface-treating agent before use.

Examples of the surface-treating agent that can be preferably used areas follows. Higher fatty acids having at least 10 carbon atoms such asstearic acid, erucic acid, palmitic acid, lauric acid and behenic acid;alkali metal salts of these higher fatty acids; sulfates of higheralcohols such as stearyl alcohol and oleyl alcohol; anionic surfactantssuch as sulfate of polyethylene glycol, amide-bonded sulfate,ester-bonded sulfate, ester-bonded sulfonate, amide-bonded sulfonate,ether-bonded sulfonate, ether-bonded alkylarylsulfonate, ester-bondedalkylarylsulfonate and amide-bonded alkylarylsulfonate; phosphate esterssuch as acid type, alkali metal salt or amine salt of a mono- or diesterof orthophosphoric acid and oleyl alcohol or stearyl alcohol or amixture of these; silane coupling agents such as vinyltrichlorosilane,vinyltrimethoxysilane, vinyltriethoxysilane,vinyltris(β-methoxyethoxy)silane,β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane,γ-glycidoxypropyltrimethoxysilane,γ-glycidoxypropylmethyldiethoxysilane, γ-glycidoxypropyltriethoxysilane,γ-methacryloxypropylmethyldimethoxysilane,γ-methacryloxypropyltrimethoxysilane,γ-methacryloxypropylmethyldiethoxysilane,γ-methacryloxypropyltriethoxysilane,N-β(aminoethyl)γ-aminopropylmethyldimethoxysilane,N-β(aminoethyl)γ-aminopropyltrimethoxysilane,N-β(aminoethyl)γ-aminopropyltriethoxysilane,γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane,N-phenyl-γ-aminopropyltrimethoxysilane andγ-mercaptopropyltrimethoxysilane; titanate-containing coupling agentssuch as isopropyltrisostearoyl titanate,isopropyltris(dioctylpyrophosphate)titanate,isopropyltri(N-aminoethyl-aminoethyl)titanate,isopropyltridecylbenzenesulfonyl titanate,tetraoctylbis(ditridecylphosphite)titanate,bis(dioctylpyrophosphate)oxyacetatetitanate,isopropyltridecylbenzenesulfonyl titanate,tetraisopropylbis(dioctylphosphite)titanate,tetra(2,2-diallyloxymethyl-1-butyl)bis-(ditridecyl)phosphitetitanate,bis-(dioctylpyrophosphate)ethylenetitanate, isopropyltrioctanoyltitanate, isopropyldimethacrylisostearoyl titanate,isopropylisostearoyldiacryl titanate,isopropyltri(dioctylphosphate)titanate, isopropyltricumylphenyltitanate, dicumylphenyloxyacetatetitanate anddiisostearoylethylenetitanate; aluminum-containing coupling agents suchas acetoalkoxyaluminum diisopropylate; triphenylphosphite,diphenyl-tridecyl phosphate, phenyl-ditridecyl phosphate,phenyl-isodecyl phosphate, tri-nonylphenyl phosphate,4,4′-butylidene-bis(3-methyl-6-tert-butylphenyl)-ditridecyl phosphite,trilauryl thiophosphite, and esters of polyhydric alcohol and fatty acidsuch as glycerin monostearate and glycerin monooleate.

When the magnesium hydroxide particles are surface-coated with the abovesurface-treating agent, the surface coating can be carried out by a wetor dry method known per se. In a wet method, for example, thesurface-treating agent in the form of a liquid or an emulsion is addedto a slurry of the magnesium hydroxide particles and these aremechanically fully mixed at a temperature up to approximately 110° C. Ina dry method, the surface-treating agent in the form of a liquid, anemulsion or a solid is added to the magnesium hydroxide particles whilethe magnesium hydroxide particles are fully stirred with a mixer such asa Henschel mixer, and these are fully mixed under heat or under no heat.While the amount of the surface-treating agent can be selected asrequired, it is preferably approximately 10% by weight or less based onthe weight of the magnesium hydroxide particles.

The surface-treated magnesium hydroxide particles may be washed withwater, dehydrated, granulated, dried, pulverized and classified asrequired, to obtain them in the form of an end product.

According to studies made by the present inventors, it has been foundthat the magnesium hydroxide particles characterized by the above aspectratio (H) can be obtained by a method in which magnesium hydroxideparticles are produced from magnesium chloride and magnesium oxide asraw materials according to a method known per se, wherein the step of areaction or hydrothermal treatment thereof is carried out in thepresence of a specific amount of at least one compound selected from thegroup consisting of an organic acid, boric acid, silicic acid andwater-soluble salts thereof. According to the present invention,therefore, the following methods (I) to (IV) are provided as methods ofproducing the above magnesium hydroxide particles.

Production Method (I):

The production method (I) is a method of producing magnesium hydroxideparticles, which comprises reacting magnesium chloride with an alkalinesubstance in an aqueous medium to produce magnesium hydroxide particles,wherein the reaction is carried out in the presence of 0.01 to 150 mol%, based on the magnesium chloride, of at least one compound (to besometimes referred to as “addition compound” hereinafter) selected fromthe group consisting of an organic acid, boric acid, silicic acid andwater-soluble salts of these.

The production method (I) has a characteristic feature in the presenceof a predetermined amount of an addition compound in a reaction systemin the known production of magnesium hydroxide particles by a reactionof magnesium chloride with an alkaline substance in an aqueous medium.Examples of the alkaline substance preferably include ammonia, alkalimetal hydroxide (typically, potassium hydroxide and sodium hydroxide)and calcium hydroxide. The amount of the alkaline substance perequivalent weight of magnesium chloride is 0.7 to 1.2 equivalentweights, preferably 0.8 to 1.1 equivalent weights.

The addition compound which is allowed to be present in the reactionsystem includes an organic acid, boric acid, silicic acid andwater-soluble salts of these, and a combination of two or more compoundsof these addition compounds may be used. The organic acid includescitric acid, tartaric acid, acetic acid and oxalic acid. Thewater-soluble salts of these organic acids, boric acid and silicic acidinclude alkali metal salt, alkaline earth metal salt, ammonium salt andorganoamine salt. The addition compound is preferably selected fromboric acid, silicic acid or water-soluble salts of these, and boric acidis particularly preferred.

The amount of the addition compound based on magnesium chloride is 0.01to 150 mol %, preferably 0.02 to 140 mol %. When the amount of theaddition compound is varied in the above range, the aspect ratio (H) ofmagnesium hydroxide particles to be obtained can be controlled asrequired. For obtaining magnesium hydroxide particles having a desiredaspect ratio (H), the kind and amount of the addition compound can bedetermined mainly by a preliminary simple experiment. Generally, whenthe amount (concentration) of the addition compound is large, magnesiumhydroxide particles having a large aspect ratio (H) tend to be obtainedas compared with a case when it is small.

The preferred amount of the addition compound differs depending upon thekind of the addition compound. When the addition compound is boric acid,silicic acid or a salt of any one of these, the amount thereof based onmagnesium chloride is preferably 0.01 to 20 mol %, particularlypreferably 0.02 to 10 mol %. When the addition compound is an organicacid or a water-soluble salt thereof, the amount thereof based onmagnesium chloride is preferably in the range of 0.1 to 140 mol %.

The above reaction of magnesium chloride with an alkaline substance inan aqueous medium (preferably, water) is carried out at a temperature inthe range, generally, of 0 to 60° C., preferably 10 to 50° C., withstirring.

By the above reaction, there can be obtained magnesium hydroxideparticles having the form intended in the present invention. However, bycarrying out hydrothermal treatment, the obtained magnesium hydroxideparticles can be formed into excellent particles having uniformity andhaving a narrow particle diameter distribution. Properly, thehydrothermal treatment is carried out in the aqueous medium at atemperature between 100° C. and 250° C., preferably between 120° C. and200° C., for 20 minutes to 10 hours, preferably, for 30 minutes to 8hours.

Production Method (II):

The production method (II) is a method of producing magnesium hydroxideparticles, which comprises reacting magnesium chloride with an alkalinesubstance in an aqueous medium to obtain a slurry of magnesium hydroxideparticles, and hydrothermally treating the slurry of magnesium hydroxideparticles to produce the magnesium hydroxide particles, wherein thehydrothermal treatment is carried out in the presence of 0.01 to 150 mol%, based on the magnesium hydroxide, of at least one compound selectedfrom the group consisting of an organic acid, boric acid, silicic acidand water-soluble salts of these.

The production process (II) substantially differs in that the additioncompound is allowed to be present in the step of hydrothermal treatmentwithout allowing the addition compound to be present in the step of thereaction in the above production process (I) (i.e., by carrying out thestep of the reaction according to a method known per se). In theproduction method (II), therefore, the reaction between magnesiumchloride and the alkaline substance is not altered with regard to thecontents explained in the above production method (I) such as the kindand amount of the alkaline substance and reaction conditions. The kindof the addition compound in the hydrothermal treatment is also asdescribed above, and further, the amount of the addition compound issubstantially not altered except that the amount is based on the formedmagnesium hydroxide.

Production Method (III):

The production method (III) is a method of producing magnesium hydroxideparticles, which comprises hydrating magnesium oxide in an aqueousmedium to produce magnesium hydroxide particles, wherein the hydrationis carried out in the presence of 0.01 to 150 mol %, based on themagnesium oxide, of at least one compound selected from the groupconsisting of an organic acid, boric acid, silicic acid andwater-soluble salts of these.

The above production method (III) has a characteristic feature in that apredetermined amount of the addition compound is allowed to be presentin a reaction system in a known method per se of producing magnesiumhydroxide particles on the basis of the hydration of magnesium oxide inan aqueous medium.

The above hydration is carried out with stirring at a temperature,generally, between 0C and 100° C., preferably 20° C. and 80° C., for 20minutes to 5 hours, preferably for 30 minutes to 4 hours.

The addition compound to be added to the reaction system is selectedfrom the same compounds as those explained in the above productionmethod (I). The amount of the addition compound based on the magnesiumoxide is 0.01 to 150 mol %, preferably 0.02 to 140 mol %. The kind andthe amount of the addition compound are preferably as those explained inthe above production method (I).

By the above hydration, magnesium hydroxide particles having the formintended in the present invention can be obtained. Further, by carryingout hydrothermal treatment, the obtained magnesium hydroxide particlescan be formed into excellent particles having uniformity and having anarrow particle diameter distribution. Desirably, the hydrothermaltreatment is carried out in the aqueous medium at a temperature between100° C. and 250° C., preferably between 120° C. and 200° C., for 20minutes to 10 hours, preferably, for 30 minutes to 8 hours.

Production Method (IV):

The production method (IV) is a method of producing magnesium hydroxideparticles, which comprises hydrating magnesium oxide in an aqueousmedium to obtain a slurry of magnesium hydroxide particles and thenhydrothermally treating the slurry, wherein the hydrothermal treatmentis carried out in the presence of 0.01 to 150 mol %, based on themagnesium hydroxide, of at least one compound selected from the groupconsisting of an organic acid, boric acid, silicic acid andwater-soluble salts of these.

The production method (IV) substantially differs in that the additioncompound is not allowed to be present in the hydration in the aboveproduction method (III) but is allowed to be present in the hydrothermaltreatment step. The hydration reaction condition in the productionmethod (IV) is therefore substantially not different from the contentsexplained in the above production method (III). Further, the additioncompound for the hydrothermal treatment can be also selected from thoseexplained in the production method (I). The amount of the additioncompound is substantially not different as that in the above productionmethod (III) except that it is based on the formed magnesium hydroxide.

The magnesium hydroxide particles intended in the present invention canbe obtained by any one of the above methods (I) to (IV). Of the abovemethods, the production methods (I) and (III) are preferred as comparedwith the other methods. In these methods in particular, if hydrothermaltreatment is carried out after the reaction, there can be obtainedparticles which are uniform in particle diameter and aspect ratio andhave a good product quality.

The obtained magnesium hydroxide particles can be formed into a powderby means such as filtration, drying after dehydration and pulverization.

The magnesium hydroxide particles featured by the above specific aspectratio in the present invention can be used in various fields, and it hasbeen found that they can be remarkably advantageously used,particularly, as a flame retardant for a synthetic resin.

According to the present invention, therefore, there is provided aflame-retardant resin composition comprising 100 parts by weight of asynthetic resin and 5 to 300 parts by weight, preferably 10 to 250 partsby weight, of magnesium hydroxide particles having an aspect ratio (H)which satisfies the following expression (1),

0.45·A·B<H<1.1·A·B  (1)

wherein H is an aspect ratio, A is an average secondary particlediameter (μm) of the particles measured by a laser diffractionscattering method and B is a specific surface area (m²/g) of theparticles measured by a BET method.

In the flame-retardant resin composition of the present invention, theabove magnesium hydroxide particles of the present invention are used asmagnesium hydroxide particles to be incorporated, and they have anaspect ratio (H) in the above (1). As already explained, the magnesiumhydroxide particles have characteristic features of the averagesecondary particle diameter (A), the BET specific surface area (B) andthe particle diameter distribution, satisfy the content of the heavymetals as impurities and further, are surface-treated. Explanations ofthese points are omitted here.

The synthetic resin used in the flame-retardant resin composition of thepresent invention can be selected from any moldable resins. Examples ofsuch resins include olefin polymers or copolymers such as polyethylene,polypropylene, polybutene-1, poly4-methylpentene, an ethylene-propylenecopolymer, an ethylene-butene-1 copolymer, an ethylene-4-methylpentenecopolymer, a propylene-butene-1 copolymer, a propylene-4-methylpentene-1copolymer, an ethylene-acrylate ester copolymer and anethylene-propylene-diene copolymer; styrene polymers or copolymers suchas polystyrene, ABS, AA, AES and AS; vinyl chloride or vinyl acetatepolymers or copolymers such as a vinyl chloride resin, a vinyl acetateresin, a vinylidene chloride resin, an ethylene-vinyl chloride copolymerand an ethylene-vinyl acetate copolymer; a phenoxy resin, a butadieneresin, a fluorine resin, a polyacetal resin, a polyamide resin, apolyurethane resin, a polyester resin, a polycarbonate resin, apolyketone resin, a methacryl resin, a diallyl phthalate resin, aphenolic resin, an epoxy resin, a melamine resin, a urea resin, andrubbers such as SBR, BR, CR, CPE, CSM, NBR, IR, IIR and a fluorinerubber.

Of these synthetic resins, thermoplastic resins are appropriate.

Examples of the thermoplastic resins are polyolefins or copolymersthereof for which the magnesium hydroxide particles are excellent inflame-retarding effect, heat-deterioration-preventing effect and theproperty of retaining mechanical strength. Specific examples thereofinclude polypropylene resins such as a polypropylene homopolymer and anethylene-propylene copolymer, polyethylene resins such as high-densitypolyethylene, low-density polyethylene, linear low-density polyethylene,ultra-low density polyethylene, EVA (ethylene vinyl acetate resin), EEA(ethylene ethyl acrylate resin), EMA (ethylene methyl acrylate copolymerresin), EAA (ethylene acrylate copolymer resin) and ultra-highpolyethylene, and polymers or copolymers of C₂-C₆ olefins (α-ethylene)such as polybutene and poly-4-methylpentene-1.

Further, the synthetic resins include thermosetting resins such as anepoxy resin, a phenolic resin, a melamine resin, an unsaturatedpolyester resin, an alkyd resin and a urea resin, and synthetic rubberssuch as EPDM, butyl rubber, isoprene rubber, SBR, NBR,chlorosulfonatedpolyethylene, NIR, urethane rubber, butadiene rubber,acryl rubber, silicone rubber and a fluorine rubber.

The resin composition of the present invention is substantiallyconstituted of a synthetic resin and the magnesium hydroxide particles,and it may further contain a small amount of a flame-retardant aid. Whena flame-retardant aid is incorporated, the amount ratio of the magnesiumhydroxide particles can be decreased, and the flame-retarding effect canbe enhanced.

The flame-retardant aid is preferably red phosphorus, a carbon powder ora mixture of these. The red phosphorus includes normal red phosphorusused for a flame retardant and others such as red phosphorussurface-coated, for example, with a thermosetting resin, a polyolefin, acarboxylate polymer, titanium oxide or a titanium-aluminum condensate.The carbon powder includes carbon black, activated carbon and graphite.The carbon black may be a product prepared by any one of an oil furnacemethod, a gas furnace method, a channeling method, a thermal method andan acetylene method.

When the flame-retardant aid is incorporated, properly, the amountthereof based on the total amount of the synthetic resin and themagnesium hydroxide particles is in the range of from 0.5 to 20% byweight, preferably 1 to 15% by weight. The resin composition of thepresent invention can be prepared by mixing the above synthetic resin,the above magnesium hydroxide particles and optionally, theflame-retardant aid in the above amount according to a method known perse.

The resin composition having anti-heat-deterioration properties andflame retardancy, provided by the present invention, may contain otherconventional additives in addition to the above components. Theadditives include an anti-oxidant, an anti-static agent, a pigment, afoaming agent, a plasticizer, a filler, a reinforcing agent, acrosslinking agent, a photostabilizer, an ultraviolet absorbent and alubricant.

According to the present invention, therefore, there can be provided aflame-retardant resin composition and a molded article which aresubstantially halogen-free. With regard to the term “substantiallyhalogen-free”, it should be understood that a very small amount of ahalogen included as a catalyst component for the production of thesynthetic resin and a very small amount of a halogen contained in theabove additives incorporated into the resin may be contained. In otherwords, it should be understood that it means that no halogenincorporated for flame retardancy is contained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1

FIG. 1 shows schematic perspective view and a side view of a regularhexagonal prism of a single crystal particle of magnesium hydroxide.

Explanations of symbols

X: Length of one side of regular hexagonal prism (μm).

Y: Thickness of prism (μm)

A: Particle diameter (μm)

EXAMPLES

The present invention will be explained with reference to Exampleshereinafter.

In Examples, (1) an average secondary particle diameter, (2) a BETspecific surface area, (3) an aspect ratio and (4) a particle sizedistribution width of magnesium hydroxide particles mean values obtainedby measurements described below.

(1) Average Secondary Particle Diameter of Magnesium Hydroxide Particles

Measured and determined with a MICROTRACK particle size analyzer SPAtype (supplied by LEEDS & amp; NORTHRUP INSTRUMENTS). A sample powder inan amount of 700 mg is added to 70 ml of water, and the mixture issupersonically dispersed (MODEL US-300, current 300 μA, supplied byNISSEI) for 3 minutes. Then, 2 to 4 ml of the dispersion is sampled andadded to a sample chamber of the above particle size analyzer retaining250 ml of degassed water, and the analyzer is operated to circulate asuspension for 2 minutes. Then, the sample is measured for a particlesize distribution. The measurement is carried out twice, and anarithmetic mean of 50% accumulated secondary particle diameters obtainedby these measurements is calculated, and used as an average secondaryparticle diameter of the sample.

(2) Specific Surface Area of Magnesium Hydroxide Particles According toBET Method

Measured by a liquid nitrogen adsorption method.

(3) Measurement of Aspect Ratio of Magnesium Hydroxide Particles

Magnesium hydroxide particles were taken as having the structure of asingle crystal and identical-particle-size-possessing regular hexagonalprism as shown in FIG. 1, X and Y were determined on the basis of foundvalues calculated values and a documented value shown in the following Ato E, and an aspect ratio was calculated.

A (μm): Average secondary particle diameter (found value)

B (m²/g): BET specific surface area (found value)

C (m²): Surface area per particle (calculated value)

D (cm³): Volume per particle (calculated value)

E (g/cm³): True specific gravity of magnesium hydroxide (documentedvalue)

H: Aspect ratio (calculated value).

A=(4x²+y²)^(½)

C=(3*3^(½)x²+6xy)*10⁻¹²

D=3/2*3^(½)x²y*10⁻¹²

E=2.38

B=C/(D*E)

H=2x/y (wherein H>1.30)

x and y are determined on the basis of the above equations, and anaspect ratio is calculated.

(4) Particle Size Distribution Width

An accumulated volume distribution stage in which a particle size was anaverage secondary particle diameter×0.3 was determined on the basis ofthe particle size distribution measured in (1). Then, a value obtainedby deducting the above value from a similarly determined accumulatedvolume distribution stage in which a particle size was an averagesecondary particle diameter×1.7 (i.e., a volume ratio of particlessatisfying the equation (2)) was taken as a particle size distributionwidth (%).

Comparative Example 1

An autoclave was charged with 400 ml of a magnesium chloride aqueoussolution (Wako Purechemical Ind., Ltd.) adjusted to a concentration of0.5 mol/L, and while the aqueous solution was stirred, 121 ml of a 3Nsodium hydroxide solution was dropwise added. The mixture was allowed toreact at room temperature (25° C.) for 30 minutes, to obtain asuspension of magnesium hydroxide particles.

The suspension was hydrothermally treated under conditions of 180° C.and 2 hours or under conditions of 160° C. and 2 hours and dehydrated,then followed by washing with water (200 ml) and drying at 105° C. for24 hours, to give magnesium hydroxide particles.

Table 1 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of each of the obtained samples.

Example 1

An autoclave was charged with 400 ml of a magnesium chloride aqueoussolution (Wako Purechemical Ind., Ltd.) adjusted to a concentration of0.5 mol/L and 0.2 mol %, based on the magnesium chloride, of boric acid(supplied by BORAX), and while the mixture was stirred, 121 ml of a 3Nsodium hydroxide solution was dropwise added. The mixture was allowed toreact at room temperature (25° C.) for 30 minutes, to obtain asuspension of magnesium hydroxide particles.

The above suspension was hydrothermally treated under the followingconditions and dehydrated, then followed by washing with water (200 ml)and drying at 105° C. for 24 hours, to give magnesium hydroxideparticles.

Samples of magnesium hydroxide particles obtained under hydrothermaltreatment conditions are referred to as Sample A-1 and Sample A-2.

Hydrothermal treatment conditions Sample name (1) 180° C., 2 hours A-1(2) 160° C., 2 hours A-2

Example 2

An autoclave was charged with 400 ml of a magnesium chloride aqueoussolution (Wako Purechemical Ind., Ltd.) adjusted to a concentration of0.5 mol/L and 1.7 mol %, based on the magnesium chloride, of boric acid(supplied by BORAX), and while the mixture was stirred, 121 ml of a 3Nsodium hydroxide solution was dropwise added. The mixture was allowed toreact at room temperature (25° C.) for 30 minutes, to obtain asuspension of magnesium hydroxide particles.

The above suspension was hydrothermally treated under the followingconditions and dehydrated, then followed by washing with water (200 ml)and drying at 105° C. for 24 hours, to give magnesium hydroxideparticles.

Samples of magnesium hydroxide particles obtained under hydrothermaltreatment conditions are referred to as Sample B-1 and Sample B-2.

Hydrothermal treatment conditions Sample name (1) 180° C., 2 hours B-1(2) 160° C., 2 hours B-2

The following Table 1 shows the average secondary particle diameter, theBET specific surface area, the particle size distribution width and theaspect ratio of each of the obtained samples.

TABLE 1 No. CEx. 1 Ex. 1 Ex. 2 Amount of added not added 0.2 1.7 boricacid (mol %) Hydrothermal treatment conditions 180° C. - 2 hours SampleNo. A-1 B-1 Particle diameter 1.34 0.94 0.70 (μm) BET specific 2.9 4.916.2 surface area (m²/g) Aspect ratio 1.75/4.27 2.07/5.07 5.10/12.47lower limit/ upper limit Aspect ratio 1.6 2.9 11.1 Particle size 74 7473 distribution width (%) Hydrothermal treatment conditions 160° C. - 2hours Sample No. A-2 B-2 Particle diameter 0.86 0.72 0.48 (μm) BETspecific 4.5 10.1 25.0 surface area (m²/g) Aspect ratio 1.74/4.263.27/8.00 5.40/13.20 lower limit/ upper limit Aspect ratio 1.6 6.2 11.9Particle size 75 73 73 distribution width (%) Ex. = Example, CEx. =Comparative Example

Comparative Example 2

An autoclave was charged with 400 ml of a magnesium chloride aqueoussolution (ion bittern) adjusted to a concentration of 0.5 mol/L, andwhile the aqueous solution was stirred, 60 ml of a slurry of calciumhydroxide (3 mol/L) was dropwise added. The mixture was allowed to reactat room temperature (25° C.) for 30 minutes, to obtain a suspension ofmagnesium hydroxide particles.

The suspension was hydrothermally treated under conditions of 160° C.and 2 hours and dehydrated, then followed by washing with water (200 ml)and drying at 105° C. for 24 hours to give magnesium hydroxideparticles.

Table 2 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

Example 3

Magnesium hydroxide particles were obtained by carrying out hydrothermaltreatment in the same manner as in Comparative Example 2 except that 0.2mol %, based on magnesium chloride, of boric acid (supplied by BORAX)was added to a magnesium chloride aqueous solution (ion bittern).

Table 2 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

Example 4

Magnesium hydroxide particles were obtained by carrying out hydrothermaltreatment in the same manner as in Comparative Example 2 except that 0.4mol %, based on magnesium chloride, of boric acid (supplied by BORAX)was added to a magnesium chloride aqueous solution (ion bittern).

Table 2 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

TABLE 2 No. CEx. 2 Ex. 3 Ex. 4 Hydrothermal treatment conditions 160°C. - 2 hours Amount of added not added 0.2 0.4 boric acid (mol %)Particle diameter 0.82 0.79 0.64 (μm) BET specific 4.7 5.1 8.3 surfacearea (m²/g) Aspect ratio 1.73/4.24 1.81/4.43 2.39/5.84 lower limit/upper limit Aspect ratio 1.5 2.0 3.8 Particle size 74 76 76 distributionwidth (%) CEx. = Comparative Example, Ex. = Example

Comparative Example 3

An autoclave was charged with 400 ml of a magnesium chloride aqueoussolution (Wako Purechemical Ind., Ltd.) adjusted to a concentration of0.5 mol/L, and while the aqueous solution was stirred, 60 ml of a slurryof calcium hydroxide (3 mol/L) was dropwise added. The mixture wasallowed to react at room temperature (25° C.) for 30 minutes, to obtaina suspension of magnesium hydroxide particles.

The suspension was hydrothermally treated under conditions of 160° C.and 2 hours and dehydrated, then followed by washing with water (200 ml)and drying at 105° C. for 24 hours, to give magnesium hydroxideparticles.

Table 3 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of each of the obtained particles.

Example 5

An autoclave was charged with 400 ml of a magnesium chloride aqueoussolution (Wako Purechemical Ind., Ltd.) adjusted to a concentration of0.5 mol/L, and while the aqueous solution was stirred, 60 ml of a slurryof calcium hydroxide (3 mol/L) was dropwise added. The mixture wasallowed to react at room temperature (25° C.) for 30 minutes, to obtaina suspension of magnesium hydroxide particles.

To the above suspension was added 12.6 g (44 mol % based on themagnesium hydroxide) of calcium acetate (Wako Purechemical Ind., Ltd.).Further, the mixture was hydrothermally treated under the followingconditions and dehydrated, then followed by washing with water (200 ml)and drying at 105° C. for 24 hours, to give magnesium hydroxideparticles. Samples of magnesium hydroxide particles obtained underhydrothermal treatment conditions are referred to as Sample C-1 andSample C-2.

Hydrothermal treatment conditions Sample name (1) 160° C., 2 hours C-1(2) 120° C., 2 hours C-2

Table 3 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of each of the obtained samples.

Example 6

An autoclave was charged with 400 ml of a magnesium chloride aqueoussolution (Wako Purechemical Ind., Ltd.) adjusted to a concentration of0.5 mol/L, and while the aqueous solution was stirred, 60 ml of a slurryof calcium hydroxide (3 mol/L) was dropwise added. The mixture wasallowed to react at room temperature (25° C.) for 30 minutes, to obtaina suspension of magnesium hydroxide particles.

To the above suspension was added 37.9 g (133 mol % based on themagnesium hydroxide) of calcium acetate (Wako Purechemical Ind., Ltd.).Further, the mixture was hydrothermally treated under the followingconditions and dehydrated, then followed by washing with water (200 ml)and drying at 105° C. for 24 hours, to give magnesium hydroxideparticles. Samples of magnesium hydroxide particles obtained underhydrothermal treatment conditions are referred to as Sample D-1 andSample D-2.

Hydrothermal treatment conditions Sample name (1) 160° C., 2 hours D-1(2) 120° C., 2 hours D-2

Table 3 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of each of the obtained samples.

TABLE 3 No. CEx. 3 Ex. 5 Ex. 6 Amount of added not added 44 133 Caacetate (mol %) Hydrothermal treatment conditions 160° C. - 2 hoursSample No. C-1 D-1 Particle diameter 1.32 0.92 0.69 (μm) BET specific2.9 5.3 7.8 surface area (m²/g) Aspect ratio 1.72/4.21 2.19/5.362.42/5.92 lower limit/ upper limit Aspect ratio 1.4 3.2 3.9 Particlesize 73 78 75 distribution width (%) Hydrothermal treatment conditions120° C. - 2 hours Sample No. C-2 D-2 Particle diameter 0.73 0.59 (μm)BET specific 6.3 8.0 surface area (m²/g) Aspect ratio 2.07/5.062.12/5.19 lower limit/ upper limit Aspect ratio 2.9 3.0 Particle size 7080 distribution width (%) Ex. = Example, CEx. = Comparative Example

Comparative Example 4

8 Grams of magnesium oxide obtained by calcining magnesium hydroxide(Kisuma 5, supplied by Kyowa Chemical Industry Co., Ltd.) at atemperature of 1,050° C. for 90 minutes was placed in an autoclavecontaining 500 ml of a calcium chloride aqueous solution having aconcentration of 1.0 mol/L (supplied by Wako Purechamical Ind., Ltd.,25° C.), and hydrated and hydrothermally treated at 170° C. for 4 hours.Then, the resultant product was dehydrated, washed with water (200 ml)and dried at 105° C. for 24 hours, to give magnesium hydroxideparticles.

Table 4 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

Example 7

Magnesium hydroxide particles were obtained by carrying out hydrationand hydrothermal treatment in the same manner as in Comparative Example4 except that 0.5 mol %, based on magnesium hydroxide, of boric acid(supplied by BORAX) was added to a calcium chloride aqueous solution(supplied by Wako Purechamical Ind., Ltd., 25° C.).

Table 4 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

Example 8

Magnesium hydroxide particles were obtained by carrying out hydrationand hydrothermal treatment in the same manner as in Comparative Example4 except that 1.0 mol %, based on magnesium hydroxide, of boric acid(supplied by BORAX) was added to a calcium chloride aqueous solution(supplied by Wako Purechamical Ind., Ltd., 25° C.).

Table 4 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

TABLE 4 No. CEx. 4 Ex. 7 Ex. 8 Amount of added not added 0.5 1.0 boricacid (mol %) Hydrothermal treatment conditions 170° C. - 2 hoursParticle diameter 1.11 0.77 0.69 (μm) BET specific 3.5 6.0 8.5 surfacearea (m²/g) Aspect ratio 1.75/4.27 2.08/5.08 2.64/6.45 lower limit/upper limit Aspect ratio 1.6 2.9 4.5 Particle size 75 76 78 distributionwidth (%) CEx. = Comparative Example, Ex. = Example

Comparative Example 5

A beaker is charged with 75 ml of a magnesium chloride aqueous solutionadjusted to a concentration of 4.0 mol/L (Brine, supplied by NEDMAG),and while the aqueous solution is stirred, 225 ml of a calcium chlorideaqueous solution adjusted to 0.127 mol/L (supplied by NEDMAG) wasdropwise added, and the mixture was stirred at room temperature for 1hour. The resultant solution was dehydrated and filtered, and a filtrateis sampled.

The total amount of the filtrate was placed in an autoclave, and whilethe filtrate was stirred, 180 ml of a sodium hydroxide aqueous solution(3N) was dropwise added. The mixture was allowed to react at roomtemperature (20° C.) for 20 minutes, to give a suspension of magnesiumhydroxide particles.

The above suspension was hydrothermally treated under conditions of 170°C. and 2 hours and dehydrated, then followed by washing with water (300ml) and drying at 105° C. for 24 hours, to give magnesium hydroxideparticles.

Table 5 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

Example 9

Magnesium hydroxide particles were obtained by carrying out hydrothermaltreatment in the same manner as in Comparative Example 5 except that 0.3mol %, based on the magnesium chloride, of boric acid (supplied byBORAX) was added to the sampled filtrate in Comparative Example 5.

Table 5 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

Example 10

Magnesium hydroxide particles were obtained by carrying out hydrothermaltreatment in the same manner as in Comparative Example 5 except that 0.4mol %, based on the magnesium chloride, of boric acid (supplied byBORAX) was added to the sampled filtrate in Comparative Example 5.

Table 5 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

TABLE 5 No. CEx. 5 Ex. 9 Ex. 10 Amount of added not added 0.3 0.4 boricacid (mol %) Hydrothermal treatment conditions 170° C. - 2 hoursParticle diameter 1.01 0.77 0.69 (μm) BET specific 3.8 6.3 7.9 surfacearea (m²/g) Aspect ratio 1.73/4.22 2.18/5.34 2.45/6.00 lower limit/upper limit Aspect ratio 1.5 3.2 4.0 Particle size 75 75 76 distributionwidth (%) CEx. = Comparative Example, Ex. = Example

Comparative Example 6

An autoclave was charged with 400 ml of a magnesium chloride aqueoussolution (ion bittern) adjusted to a concentration of 0.5 mol/L, andwhile the aqueous solution was stirred, 60 ml of a slurry of calciumhydroxide (3 mol/L) was dropwise added. The mixture was allowed to reactat room temperature (25° C.) for 30 minutes, to obtain a suspension ofmagnesium hydroxide particles.

The suspension was hydrothermally treated under conditions of 170° C.and 2 hours and dehydrated, then followed by washing with water (200 ml)and drying at 105° C. for 24 hours to give magnesium hydroxideparticles.

Table 6 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

Example 11

Magnesium hydroxide particles were obtained by carrying out hydrothermaltreatment in the same manner as in Comparative Example 6 except that0.02 mol %, based on magnesium hydroxide, of water glass adjusted toSiO₂=1.084 g/l (Wako Purechemical Ind., Ltd. ) was added to an obtainedsuspension in Comparative Example 6.

Table 6 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

Example 12

Magnesium hydroxide particles were obtained by carrying out hydrothermaltreatment in the same manner as in Comparative Example 6 except that 0.05 mol %, based on magnesium hydroxide, of water glass adjusted toSiO₂=1.084 g/l (Wako Purechemical Ind., Ltd.) was added to an obtainedsuspension in Comparative Example 6.

Table 6 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

TABLE 6 No. CEx. 6 Ex. 11 Ex. 12 Amount of added not added 0.02 0.05water glass (mol %) Hydrothermal treatment conditions 170° C. - 2 hoursParticle diameter 1.04 0.75 0.70 (μm) BET specific 3.7 7.2 8.8 surfacearea (m²/g) Aspect ratio 1.73/4.23 2.43/5.94 2.77/6.78 lower limit/upper limit Aspect ratio 1.5 3.9 4.9 Particle size 75 77 76 distributionwidth (%) CEx. = Comparative Example, Ex. = Example

Comparative Example 7

An autoclave was charged with 400 ml of a magnesium chloride aqueoussolution (ion bittern) adjusted to a concentration of 0.5 mol/L, andwhile the aqueous solution was stirred, 60 ml of a slurry of calciumhydroxide (3 mol/L) was dropwise added. The mixture was allowed to reactat room temperature (25° C.) for 30 minutes, to obtain a suspension ofmagnesium hydroxide particles.

The suspension was hydrothermally treated under conditions of 170° C.and 2 hours and dehydrated, then followed by washing with water (200 ml)and drying at 105° C. for 24 hours to give magnesium hydroxideparticles.

Table 7 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

Example 13

Magnesium hydroxide particles were obtained by carrying out hydrothermaltreatment in the same manner as in Comparative Example 7 except that1.206 g (5 mol % based on magnesium hydroxide) of sodium oxalate (WakoPurechemical Ind., Ltd.) was added to an obtained suspension inComparative Example 7.

Table 7 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

Example 14

Magnesium hydroxide particles were obtained by carrying out hydrothermaltreatment in the same manner as in Comparative Example 7 except that3.618 g (15 mol % based on magnesium hydroxide) of sodium oxalate (WakoPurechemical Ind., Ltd.) was added to an obtained suspension inComparative Example 7.

Table 7 to be described later shows the average secondary particlediameter, the BET specific surface area, the particle size distributionwidth and the aspect ratio of the obtained sample.

TABLE 7 No. CEx. 7 Ex. 13 Ex. 14 Amount of added not added 5 15 sodiumoxalate (mol %) Hydrothermal treatment conditions 170° C. - 2 hoursParticle diameter 1.38 0.86 0.81 (μm) BET specific 2.8 6.0 7.9 surfacearea (m²/g) Aspect ratio 1.74/4.25 2.32/5.68 2.88/7.04 lower limit/upper limit Aspect ratio 1.6 3.6 5.2 Particle size 75 75 76 distributionwidth (%) CEx. = Comparative Example, Ex. = Example

Examples 15 and 16, and Comparative Example 8

The magnesium hydroxide particles obtained in the above ComparativeExample 1, Example 1 and Example 2 (samples obtained by hydrothermaltreatment at 180° C. for 2 hours) were surface-treated withisopropyltruisostearoyl titanate such that it adhered in an amount of 2%by weight based on the magnesium hydroxide particles. Table 8 showsmeasurement values before the surface treatment.

Then, 150 parts by weight of the obtained magnesium hydroxide particles,100 parts by weight of an ethylene-vinyl acetate copolymer resin and 0.2part by weight of an antioxidant (Irganox 1010, supplied by Ciba SpecialChemicals) were mixed to prepare three mixtures in an amount of 10 kgeach. Each mixture was separately kneaded with a single-screw extruderat 220° C. to prepare kneaded pellets. Further, the pellets were formedinto a sheet form with a compression molding machine at 220° C.,obtained sheets were processed into test pieces, and the test pieceswere measured for physical properties and flame retardancy. Table 8shows the results.

In Table 8, the total content of heavy metals shows a total content ofheavy metals (Fe, Mn, Co, Cr, Cu, V and Ni) in the magnesium hydroxideparticles.

TABLE 8 CEx. 8. Ex. 15 Ex. 16 Physical properties of magnesium hydroxideparticles Average secondary particle diameter 1.34 0.94 0.70 (μm) BETspecific surface area (m²/g) 2.9 4.9 16.2 Aspect ratio 1.6 2.9 11.1Lower limit and upper limit of 1.75/4.27 2.07/5.07 5.10/12.5 aspectratio Ratio of amount within particle size 74 74 73 distribution width(%) Total content of heavy metals (%) 0.0019> 0.0018> 0.0010> Physicalproperties of resin composition Appearance good good good Tensilestrength (kgf/mm²) 0.48 0.76 1.05 Flame retardancy test UL-94V V-0 V-0V-0 (⅛ inch) Number of days before whitening 36 36 35 (days) Weight lossby 10% by weight (hour) 831 825 810 CEx. = Comparative Example, Ex. =Example

Comparative Example 9

8 Grams of magnesium oxide prepared by calcining magnesium hydroxide(total content of heavy metals 0.0351 wt %) at a temperature of 1,050°C. for 90 minutes was placed in an autoclave containing 500 ml of acalcium chloride aqueous solution having a concentration of 1.0 mol/L(Wako Purechemical Ind., Ltd., 25° C.), hydrated and hydrothermallytreated at 170° C. for 4 hours and dehydrated, then followed by washingwith water (200 ml) and drying at 105° C. for 24 hours, to obtainmagnesium hydroxide particles.

The obtained sample was measured or determined for an average secondaryparticle diameter, a BET specific surface area, a particle sizedistribution width and aspect ratio.

Examples 17 and 18, and Comparative Example 10

The magnesium hydroxide particles obtained in the above ComparativeExample 9, Example 7 and Example 8 were respectively emulsified inwater. Emulsification products were heated to 80° C. and surface-treatedby adding 2% by weight, based on the magnesium hydroxide particles, ofstearic acid. Then, the products were dehydrated, dried and pulverized.Each sample before the surface treatment was measured for an averagesecondary particle diameter, a BET specific surface area, a particlesize distribution width and an aspect ratio. Table 9 shows the results.

150 Parts by weight of the obtained magnesium hydroxide particles, 100parts by weight of a polypropylene resin and 1 part by weight of anantioxidant were mixed to prepare mixtures in an amount of 10 kg each.The mixtures were respectively kneaded with a twin-screw extruder toprepare kneaded pellets. Further, the pellets were injection-molded toobtain test pieces, and then the test pieces were measured for physicalproperties, flame retardancy and thermal stability. Table 9 shows theresults.

(i) Method of preparation of the test pieces, (ii) measurement ofthermal stability, (iii) tensile strength test and (iv) flexuralstrength and flexural modulus test were as follows.

(i) Method of Preparation of the Test Pieces

Surface-treated magnesium hydroxide particles of each sample werepre-dried under conditions of 105° C.×16 hours and further underconditions of 120° C.×2 hours for removing adhering water, and thenkneaded in a twin-screw extruder together with a resin (polypropylene)and an antioxidant at 230° C., and each sample was further dried underconditions of 120° C.×2 hours and molded with an injection moldingmachine at 230° C.

Twin-screw extruder; supplied by Plastic Kogaku Kenkyusho

BT-30-S2-30-L

Injection molding machine; supplied by Nissin Jushi Kogyo K.K.

FS 120S 18A SE

(ii) Measurement of Thermal Stability

Machine: Gear Open GPHH-100, supplied by Tabaiespec

Setting conditions: 150° C., damper opening degree 50%

Test pieces were used as sets of two test pieces each. Each set waspressed with paper on both sides in an upper portion, held with a crip,suspended from a rotary ring and taken off with the passage of time.

Test pieces: {fraction (1/12)} inch

Evaluation: A time period taken before whitening was observed on a testpiece was used as an index for heat deterioration. Further, a timeperiod taken before a weight loss by 10% by weight of the test piece at150° C. took place.

(iii) Tensil Strength Test

According to JIS K 7113.

(iv) Flexural Strength, Flexural Modulus Test

According to JIS K 7203.

TABLE 9 CEx. 9 Ex. 17 Ex. 18 Physical properties of magnesium hydroxideparticles Average secondary particle diameter 0.63 0.77 0.69 (μm) BETspecific surface area (m²/g) 20.2 6.0 8.5 Aspect ratio 12.8 2.9 4.5Lower limit and upper limit of 5.73/14.00 2.08/5.08 2.64/6.45 aspectratio Ratio of amount within particle size 75 76 78 distribution width(%) Total content of heavy metals (%) 0.0362> 0.0018> 0.0020> Physicalproperties of resin composition Appearance (visually observed) poor goodgood Tensile strength (kgf/mm²) 1.29 1.78 1.89 Flexural strength(kgf/mm²) 3.02 3.64 3.77 Flexural modulus (kgf/mm²) 453 515 638 Flameretardancy test UL-94V V-0 V-0 V-0 (⅛ inch) Number of days beforewhitening 8 37 35 (days) Weight loss by 10% by weight (hour) 210 827 830CEx. = Comparative Example, Ex. = Example

Comparative Example 11

An autoclave was charged with 400 ml of a magnesium chloride aqueoussolution (ion bittern) adjusted to a concentration of 0.5 mol/L, andwhile the aqueous solution was stirred, 60 ml of a slurry of calciumhydroxide (3 mol/L) was dropwise added. The mixture was allowed to reactat room temperature (25° C.) for 30 minutes, to obtain a suspension ofmagnesium hydroxide particles.

The suspension was hydrothermally treated under conditions of 160° C.and 2 hours and dehydrated, then followed by washing with water (200 ml)and drying at 105° C. for 24 hours to give magnesium hydroxideparticles.

The obtained magnesium hydroxide particles were measured or determinedfor an average secondary particle diameter, a BET specific surface area,a particle size distribution width and an aspect ratio.

Examples 19 and 20, and Comparative Example 12

The magnesium hydroxide particles obtained in the above ComparativeExample 11, Example 5 and Example 6 (hydrothermal treatment conditions160° C.—2 hours) were respectively washed with water and emulsified.Each emulsification product was heated to 80° C., and surface-treated byadding 3% by weight, based on the magnesium hydroxide, of sodium oleate,followed by dehydration, drying and pulverization. The obtained samplesbefore the surface-treatment were measured for an average secondaryparticle diameter, a BET specific surface area, a particle sizedistribution width and an aspect ratio. Table 10 shows the results.

180 Parts by weight of the obtained magnesium hydroxide particles, 100parts by weight of a polypropylene resin and 1 part by weight of anantioxidant were mixed to prepare three mixtures in an amount of 10 kgeach. Each mixture was separately kneaded with a single-screw extruderto prepare kneaded pellets. Further, the pellets were injection-moldedto prepare test pieces, and the test pieces were measured for physicalproperties and flame retardancy. Table 10 shows the results.

TABLE 10 CEx. 9 Ex. 17 Ex. 18 Physical properties of magnesium hydroxideparticles Average secondary particle 0.55 0.92 0.69 diameter (μm) BETspecific surface area (m²/g) 47.9 5.3 7.8 Aspect ratio 29.0 3.2 3.9Lower limit and upper limit of 11.86/28.98 2.19/5.36 2.42/5.92 aspectratio Ratio of amount within particle 72 78 75 size distribution width(%) Total content of heavy metals (%) 0.2102> 0.0040> 0.0034> Physicalproperties of resin composition Appearance (visually observed) poor goodgood Tensile strength (kgf/mm²) 1.21 1.82 1.84 Flexural strength(kgf/mm²) 2.88 3.62 3.73 Flexural modulus (kgf/mm²) 425 597 621 Flameretardancy test UL-94V V-0 V-0 V-0 ({fraction (1/12)}inch) Number ofdays before whitening 4 25 27 (days) Weight loss by 10% by weight 120620 670 (hour) CEx. = Comparative Example, Ex. = Example

Example 21

The following resin compositions (1) to (3) were prepared, and testpieces were prepared therefrom in the same manner as in Examples 15 and16 and evaluated for flame retardancy. When the resin was Nylon 6 in(1), kneading and injection molding were carried out at 250° C. As aresult, the test pieces from the resin compositions (1) and (2) showed aflame retardancy of V-0 under UL94-V {fraction (1/16)} inch. The testpiece from the resin composition (3) showed a flame retardancy of V-0under UL94-V ⅛ inch.

(1) Resin Composition

190 parts by weight, Magnesium hydroxide particles (test sample A-1 inExample 1)

100 parts by weight, Nylon 6 (specific gravity 1.14, injection moldinggrade)

0.2 part by weight, Antioxidant (Irganox 1096, Ciba Special Chemicals)

(2) Resin Composition

210 parts by weight, Magnesium hydroxide particles (test sample A-1 inExample 1)

100 parts by weight, High-density polyethylene (MFI 5.0 g/10 minutes,injection molding grade)

0.25 part by weight, Antioxidant (Irganox 1010, Ciba Special Chemicals)

0.25 part by weight, Antioxidant (DLTP, supplied by YoshitomiPharmaceuticals)

(3) Resin Composition

10 parts by weight, Magnesium hydroxide particles (test sample B-1 inExample 2 which was altered by surface treatment with 0.3%, based onmagnesium hydroxide particles, of γ-aminopropyltrimethoxysilane)

10 part by weight, Red phosphorus (Novaexcel 140, supplied by Rin KagakuKogyo)

5 parts by weight, Carbon black (oil furnace method, FEF)

95 parts by weight, ABS resin (MFR 25 g/10 minutes, impact resistancegrade)

5 parts by weight, Nylon 6 (specific gravity 1.14, injection moldinggrade)

0.2 part by weight, Antioxidant (Irganox 1010, Ciba Special Chemicals)

Example 22

The following resin composition was prepared and masticated with an ovenroll at 70° C., and after one day, the mastication product wasvulcanized at 160° C. for 30 minutes, to give a plate having a thicknessof ⅛ inch. Then, a ⅛ inch thick test piece for UL94-V test was preparedfrom the plate. The test piece was subjected to the UL94-V test. Thetest result was that the test piece had a flame retardancy of V-1.

Composition

100 parts by weight, EPDM rubber (ethylene/propylene ratio=50/50 mol)

170 parts by weight, Magnesium hydroxide particles (test sample A-1 inExample 1)

3 parts by weight, Dicumyl peroxide

0.5 part by weight, poly(2,2,4-trimethyl-1,2-dihydroquinoline)

1 part by weight, Silane coupling agent (A-172, supplied by NipponUnitika)

1 part by weight, Stearic acid

1 part by weight, Sulfur

Example 23

The following composition was prepared and kneaded with a kneader atapproximately 30° C. The kneaded mixture was cured at 90° C. for 15minutes, to give a ⅛ inch thick plate. Then, a ⅛ inch thick test piecefor UL94-V test was prepared from the plate. The test piece wassubjected to the UL94-V test. The test result was that the test piecehad a flame retardancy of V-0.

Composition

100 parts by weight, epoxy resin (specific gravity 1.17)

100 parts by weight, magnesium hydroxide particles (test sample A-1 inexample 1)

5 parts by weight, red phosphorus (novaexcel 140, supplied by rinkagaku)

1 part by weight, carbon black (oil furnace method, FFF)

10 parts by weight, curing agent (HY951, supplied by Ciba SpecialChemicals)

1 part by weight, Stearic acid

0.2 part by weight, Irganox 1010

What is claimed is:
 1. A method of producing magnesium hydroxideparticles having a hexagonal crystal form and having an aspect ratio (H)which satisfies the following expression (I), 0.45·A·B<H<1.1·A·B  (I)wherein H is an aspect ratio, A is an average secondary particlediameter (μm) of all of the particles measured by a laser diffractionscattering method and B is a specific surface area (m²/g) of all of theparticles measured by a BET method, which comprises reacting magnesiumchloride with an alkaline substance in an aqueous medium to producemagnesium hydroxide particles, wherein the reaction is carried out inthe presence of 0.01 to 150 mol %, based on the magnesium chloride, ofat least one compound selected from the group consisting of boric acid,silicic acid and water-soluble salts of these and then the magnesiumhydroxide particles obtained by the reaction are further hydrothermallytreated.
 2. A method of producing magnesium hydroxide particles having ahexagonal crystal form and having an aspect ratio (H) which satisfiesthe following expression (I), 0.45·A·B<H<1.1·A·B  (I) wherein H is anaspect ratio, A is an average secondary particle diameter (μm) of all ofthe particles measured by a laser diffraction scattering method and B isa specific surface area (m²/g) of all of the particles measured by a BETmethod, which comprises reacting magnesium chloride with an alkalinesubstance in an aqueous medium to obtain a slurry of magnesium hydroxideparticles, and hydrothermally treating the slurry to produce themagnesium hydroxide particles, wherein the hydrothermal treatment iscarried out in the presence of 0.01 to 150 mol %, based on the magnesiumhydroxide, of at least one compound selected from the group consistingof boric acid, silicic acid and water-soluble salts of these.
 3. Amethod of producing magnesium hydroxide particles having a hexagonalcrystal form and having an aspect ratio (H) which satisfies thefollowing expression (I), 0.45·A·B<H<1.1 19 A·B  (I) wherein H is anaspect ratio, A is an average secondary particle diameter (μm) of all ofthe particles measured by a laser diffraction scattering method and B isa specific surface area (m²/g) of all of the particles measured by a BETmethod, which comprises hydrating magnesium oxide in an aqueous mediumto produce magnesium hydroxide particles, wherein the hydration iscarried out in the presence of 0.01 to 150 mol %, based on the magnesiumoxide, of at least one compound selected from the group consisting ofboric acid, silicic acid and water-soluble salts of these and then themagnesium hydroxide particles obtained by the hydration are furtherhydrothermally treated.
 4. A method of producing magnesium hydroxideparticles having a hexagonal crystal form and having an aspect ratio (H)which satisfies the following expression (I), 0.45·A·B<H<1.1·A·B  (I)wherein H is an aspect ratio, A is an average secondary particlediameter (μm) of all of the particles measured by a laser diffractionscattering method and B is a specific surface area (m²/g) of all of theparticles measured by a BET method, which comprises hydrating magnesiumoxide in an aqueous medium to obtain a slurry of magnesium hydroxideparticles and then hydrothermally treating the slurry to produce themagnesium hydroxide particles, wherein the hydrothermal treatment iscarried out in the presence of 0.01 to 150 mol %, based on the magnesiumhydroxide, of at least one compound selected from the group consistingof boric acid, silicic acid and water-soluble salts of these.